The present invention relates to a multi-station transmission method which is used in a mobile communication system and a broadcasting system to transmit the same signal from a plurality of stations with a view to holding the continuity of a signal in the vicinity of the one boundary, and the invention also pertains to a receiver therefor.
In radio communication, especially in mobile communication, it is necessary to implement many channels in a limited frequency is an important technique For example, a cellular system has been employed in mobile communication, in the cellular system, the service area is divided into plurality of cells, i.e., zones, and different frequencies are assigned to the cells to prevent interference between them, but in a bid to promote the spatial reuse of frequency, it is customary to assign the same frequency to cells that are far enough apart not to interfere with each other. Such a cellular system requires a handoff capability which enables the mobile station to keep up conversation when it moves from one cell to another, that is, from one zone to another.
FIG. 1 shows the principles of a conventional zone switching scheme. Let it be assured that zones Z1 and Z2 covered by base stations BS1 and BS2 are adjacent and that a mobile station M is now moving across the boundary of the zones Z1 and Z2 in a direction from the base station BS1 toward the base station BS2. Forward signals to be sent from the base stations BS1 and BS2 to the mobile station M are transmitted from a switching center 13 to the two base stations BS1 and BS2 which are switched from the one to the other. A forward radio channel to the mobile station M is set first by a first channel CH1 via the base station BS1. When the field intensity of the first channel CH1 decreases with the movement of the mobile station M, a second channel CH2 is set as the forward radio channel via the base station BS2, while at the same time the first channel CH1 is cut off. Since an access channel is usually set up by a FDMA (Frequency Division Multiple Access) or TDMA (Time Division Multiple Access) scheme, the same channel cannot be used in adjacent zones. Hence, the two channels CH1 and CH2 use different carrier frequencies. On this account, the channels cannot continuously be switched from one to the other, inevitably resulting in a momentary interruption during switching. In voice communication this interruption can be made sufficiently short as not to seriously affect the speech quality, but in multimedia transmission such as visual or data communication the momentary interruption causes significant quality deterioration because of high-speed transmission of digital signals in many cases.
On the other hand, in zone switching by a CDMA (Code Division Multiple Access) scheme, a spreading code is used for channel setting and the same carrier frequency is used in common to adjacent zones. Then when a multi-station transmission is carried out using different spreading codes for the adjacent zones, signals from two base stations can easily be received and interruption-free reception is possible. However, this method is inherent to the CDMA scheme and cannot be applied to the FDMA and TDMA schemes. Furthermore, to identify the respective channel defined by the spreading code, it is necessary to use a different spreading code for each channel. There is another method which, instead of changing the spreading code, shifts its timing to avoid overlapping of pulses detected by the two inverse transformation circuits for despreading of the two base station is, but highly accurate transmission timing must be provided between the base stations.
On the other hand, a forward control signal in the mobile communication system needs to call up mobile stations over a wide area. To cover a wide area with a low transmission output, a multi-station transmission system is needed which divides the area into a plurality of zones and transmits the same signal to the respective zones.
In the multi-station transmission system, even if the same signals are transmitted from the respective zones, they do not completely match in terms of transmission carrier frequency and transmission data timing, posing a problem that the signals from the plurality of zones interfere with each other fit the boundary between them. To solve this problem, frequency offset type transmitter diversity or the like has been employed. This method is one that offsets the transmitter carrier frequency of each zone in the range of from xc2xd to xc2xc of the modulation band and receives the frequency offset signals by a differential detector at the receiving side, thus enabling a diversity, reception. However, this method has a disadvantage in that if the data timing is not the same, interference will occur and the frequency offsetting enlarges the receiving band width correspondingly, making it hard to implement a narrow-band communication
An object of the present invention is to provide a multistation transmission method and a receiver therefor which, regardless of the access scheme used, allow zone switching free from signal discontinuity and enable simultaneous reception of identical signals from a 1 plurality of base stations without widening the receiving band, thereby implementing highly reliable reception based on the diversity effect.
In a mobile communication system in which the service area is broken up into a plurality of zones each having a base station and a mobile station performs communication via the base station of its visited zone, the multi-station transmission method according to the present invention, transmits the same signal from the base station of the visited zone and the base station of at least one adjacent zone when the mobile station moves across the boundary between its visited zone and the adjacent zone, the method comprising the following steps
(a) the same forward signal sequence destined to the mobile station is transmitted to N base stations including the base station of the mobile station""s visited zone and the base station of at least one adjacent zone, N being an integer equal to or greater than 2;
(b) each of the N base stations each converts the forward signal sequence to a transmission signal sequence and adds predetermined pseudo-orthogonal training signals to the transmission signal sequence for each frame to generate a framed signal sequence;
(c) the N base stations each send the framed signal sequence by a transmission radio wave of the same channel; and
(d) said mobile station receives the transmitted radio wave from each base station, then separates It into N transmitted signal sequences from the N base stations through utilization of the previously known N training signals and obtains a desired received signal sequence from the transmitted signal sequences.
The receiver according to the present invention, is provided with: separating means which separates received waves of the same channel by their training signals into a plurality of signal sequences corresponding to the training signals, respectively; inverse transformation means which subject these separated signal sequences to transformation inverse from that effected thereon at the transmitting sides to restore the original signal sequences; and means which outputs that one of the restored signal sequences which is high in reliability at the time of separation.